Preheat oven to 350F. Mash the bananas in a bowl. Add melted butter, egg and vanilla and stir. In a separate bowl combine dry ingredients. Add dry ingredients to banana mixture and stir well with a wooden spoon.
In a separate bowl, mix about half of the batter with the melted chocolate chips. Spoon chocolate batter alternately with plain batter into an 8 1/2 x 4 1/2-inch loaf pan coated with cooking spray. With a knife, swirl batters together.
Bake until a toothpick stuck into the bread comes out clean, 55 to 60 minutes. Slice and serve.
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